ament_vendor() does not install vendor project to correct path

Hi,

I'm building `ros2/rosbag2/zstd_vendor` and the cmake file is like:
```cmake
ament_vendor(zstd_vendor
SATISFIED ${zstd_FOUND}
VCS_URL https://github.com/facebook/zstd.git
VCS_VERSION v1.4.8
SOURCE_SUBDIR build/cmake
CMAKE_ARGS
-DZSTD_BUILD_STATIC:BOOL=OFF
-DZSTD_BUILD_SHARED:BOOL=ON
-DZSTD_BUILD_PROGRAMS:BOOL=OFF
)
```
but the current result is that `bin/include/lib` are installed at `C:\ros\rolling\install\opt\zstd_vendor`
but by `colcon build --merge-install`, I think those 3 sub folders should be installed to `C:\ros\rolling\install`.